GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 79 



In Peekskill valley the formations are very distinct, and the whole 

 Hudson River- Wappinger-Poughquag series is fully represented. 

 It is clear that a tilted and down-dropped block forms the whole 

 floor of this valley, the formation standing practically vertical on the 

 east side, the principal fault movement being recorded on the west 

 side. Exploratory work has been carried on in connection with the 

 Catskill aqueduct investigation which gives accurate data on the 

 location and attitude of the rocks across the whole valley, even where 

 they are heavily covered with drift. The major structure, therefore, 

 is very well known and the main facts on those explorations may be 

 found in N. Y. State Mus.. Bui. 146. 



The striking thing in this isolated block of Cambro-Ordovician 

 sediments is the close isoclinal folding which seems to have tripled 

 the thickness of the limestones, while the block shows down-faulting 

 of a sort that is difficult to associate with the type of folding repre- 

 sented. It seems necessary, therefore, to invoke the aid of deforma- 

 tion of two periods, first a close folding of the Appalachian type 

 which probably took place in Appalachian time. Second, a down- 

 faulting and tilting of the block which belongs to the period of Trias- 

 sic deformation. The chief deformation zones of these two periods 

 are nearly parallel and this causes confusion as to which period really 

 caused the folding. Also it is possible that some folding and shear- 

 ing accompanied the block faulting. It does not seem reasonable, 

 however, considering the simplicity of the typical Triassic blocks 

 to charge much of the folding of this isolated block to the deforma- 

 tion of Triassic time. 



It is entirely possible that block faulting preceded the Triassic 

 period of deposition also and thus outlined some of the principal 

 areas of that formation. The great Triassic block which carries 

 many thousands of feet of sediments on the west side of the Hudson 

 in northern New Jersey and adjacent portions of New York, would 

 be such a case. At the extreme northeast point of this acute-angled 

 block the unique igneous intrusive masses known as the Cortlandt 

 series are located. This intrusion seems to have followed the weak- 

 ness developed at this angle where several faults converge. 



It is not possible to say how many dislocations belong to the Trias- 

 sic deformation. Undoubtedly there are some examples in the 

 Highlands and also north of the Highlands, but nothing of equal 

 prominence to this Peekskill Hollow fault presents data of decisive 

 enough character to warrant description. 



Possible other types. The sudden ending of the series of crystal- 



